1. Field of the Invention
The present invention pertains to the commercial manufacture of polyethylene terephthalate (“PET”) polymers.
2. Background Art
PET has numerous uses, principle among which are for films, fibers, and food containers. Despite the stringent matrix of properties required for such uses, particularly for food packaging, some PET has become a commodity polymer. Commercial production of PET is energy intensive, and therefore even relatively small improvements in energy consumption are of considerable commercial value.
The production of PET (inclusive of copolymers) begins with an esterification step where the dicarboxylic acid component, predominantly terephthalic acid, is slurried in ethylene glycol and heated to produce a mixture of oligomers of a low degree of polymerization. This “esterification” step may be followed by a further “oligomerization” or “prepolymer” step, where a higher degree of polymerization is obtained. The product still has a very low molecular weight at this stage.
The previously described steps are then followed by a polycondensation. The polycondensation is catalyzed by metal compounds such as Sb, Ti, Ge, Sn, etc. Polycondensation occurs at relatively high temperature, generally in the range of 280–300° C., under vacuum, water and ethylene glycol produced by the condensation being removed. The polymer at the end of polycondensation has an inherent viscosity generally in the range of 0.4 to 0.65, corresponding to a molecular weight too low for many applications.
Commercial production of PET polyesters has required a subsequent post-polymerization in the solid state, termed “solid stating.” In this stage of the process, the PET granules are heated in inert gas, preferably nitrogen, at temperatures below the melt temperature, i.e. from 210–220° C. in many cases. Solid stating is complicated by the fact that most PET polymers, following extrusion from the melt and pelletizing, are substantially amorphous. In order to prevent the pellets from sintering and agglomerating in the solid stater, the pellets are first crystallized over a period of 30 to 90 minutes at a lower temperature, e.g. 160–190° C., typically in a flow of inert gas or air. It should be noted that “solid stating” herein refers to the solid state polycondensation per se, and not to the combined processes of crystallization and solid state polycondensation. These procedures are well known to those skilled in the art, as evidenced by U.S. Pat. Nos. 5,597,891 and 6,159,406.
In the conventional PET process, the polymer is extruded directly from the polycondensation reactor into strands. The hot, extruded strands are contacted with cool water prior to chopping into pellets, dried, and stored into silos prior to crystallizing. Conventional pelletizing processes as well as a pelletizing process wherein strands are stretched prior to pelletizing are disclosed in U.S. Pat. No. 5,310,515. Conventional wisdom dictates that at least the surface of the pellets must be cooled to 20° to 30° C. to avoid sintering during storage. During storage, heat from the hotter interior of the pellets is distributed throughout the pellets. Thus, warm pellets, i.e. pellets whose exterior is significantly higher than 20–30° C. might agglomerate during storage following temperature equilibration. In addition to the decrease in temperature brought about by contact with water, the pellets can be further cooled to the desired temperature with cool air or nitrogen. The pellets are stored, and then subsequently reheated to the desired crystallization temperature. These steps of heating, cooling, and reheating entail a significant energy penalty in an already energy intensive process.